If a tube needs replacing, it might still be lit up right?

How could I check to determine which tube needs replacing. This is for a Hammond organ from the 50's/60's, it has 11 tubes and isn't designed to blow through tubes the way guitar amps are so replacing all of them isn't cheap or necessary. I have an idea of which tube is causing the issue, but I'm also not sure it's only a tube issue, hence the need to test the tubes.
what are the symptoms? nasty feedback? distorted sound?
1978 Peavey T-40 -> Ampeg Micro-VR - > Ampeg SVT210AV + Ampeg SVT-15E
Just because a tube is lit, doesn't mean that it's good. Your idea of testing the tubes is a good one - unless you know which tube is causing the problem.
sounds like you're dealing with dual triode preamp tubes?

i'd buy 1 or 2 tubes (most likely you are dealing with 12ax-7's) and replace the tubes in the organ one at a time with the new tubes until the problem tube is identified. i have done this before and it works fine.

the procedure would be like:

1) take out V1 tube
2) replace with new tube
3) test keyboard

if it that is not the problem tube then take the new tube back out, put the old tube back in and go back to step 1 but replace the V2 tube, etc, until you have tested all the slots
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
Last edited by gumbilicious at Aug 13, 2011,
i don't know if this would work but gently hit the sides and see if there microphonic
that's what they do for valve amps.
The sound is just very weak. It plays, just very very quietly. Which has made me wonder if one of the tubes has gone. The tubes are:

5U4 rectifier
6BA6 bass pedal preamp
2x 12AU7 and 2x 6C4 percussion amp (i think)
6AU6 clean channel preamp
6AU6 and 12AU7 vibrato channel preamp
12AX7 "intermediate amp"
2x 6V6 power amp

Changing the volume expression pedal doesn't affect the volume so at first I thought it might be that, but I've pretty much ruled that out at this point.